What's the deal with the Dana 61?
With mileage and emissions concerns rising, at some point Dodge and / or Ford decided they wanted to be able to offer gears higher than the current highest ratio of 3.54 available in the D60. However, because of the design of the D60 the pinion head could not be made big enough to achieve a higher ratio. Hence, the birth of the Dana 61, which has a different casting that moves the pinion over 0.25"-0.437" (conflicting reports) - this allowed for a 3.07 axle ratio. The Dana 61 also uses 3.5" axle tubes. Other than that, it is very similar to the Dana 60. The Dana 61 was common in Dodge front ends and Ford van rears.

The difficulty with the Dana 61 is very limited gear and locker choices. However, you can use a combination of ring gear spacer and thick gear set to allow the use of differentials and gear sets intended for the Dana 60.

Slagburn reports: to run a 4.88 I had to go with series 3 carrier, series 1 being 4.10 and down, series 2 being 4.56 and up, then the series 3. PLUS a .160" spacer. That's the problem, nobody makes a full case locker for a 61, but it could be doable with a thick ring gearset AND a spacer.

Reider racing< Randy's R&P, and other sources make ring gear spacers for the Dana 61 .